Swift: Multithreading Issue Getting Data From API - swift

Question

I'm having a problem where sometimes I can't successfully decode a json file due to a slow networking call. I have a list of stocks and I'm trying to successfully decode each one before the next stock gets decoded.
For example, I would have a list of 4 stocks but 3 would be successful decoded but 1 won't. The one that fails is also random. When I print out the url for the one that fails, the url and json file is correct yet I get an error of it not reading because its on a wrong format.
The list of stocks are retrieved through Firebase and after I receive them, I have a completion handler that tries to make a network call to the server. The reason why I added Firestore code here is because when I put a stop point at when case is successful, I notice that its hitting Thread 5 out of 14 Threads. Is having this many threads common? I know it's a threading issue but am having such a huge problem identifying where I should do dispatchGroups. Any help and clarifications would be much appreciated!
APIManager
private var stocks = [Stock]()
func getStockList( for symbols: [Stock], completion: ((Result<[Stock]>) -> Void)?) {
let dispatchGroup = DispatchGroup()
for symbol in symbols {
var urlComponents = URLComponents()
urlComponents.scheme = "https"
urlComponents.host = APIManager.baseAPIURL
urlComponents.path = "\(APIManager.baseRelativePath)/market/batch"
//URL parameters
let token = URLQueryItem(name: "token", value: "fakeToken")
let symbolsItem = URLQueryItem(name: "symbols", value: symbol.symbol)
let typesItem = URLQueryItem(name: "types", value: "quote")
urlComponents.queryItems = [token, symbolsItem, typesItem]
guard let url = urlComponents.url else { fatalError("Could not create URL from components") }
var request = URLRequest(url: url)
request.httpMethod = "GET"
let config = URLSessionConfiguration.default
let session = URLSession(configuration: config)
dispatchGroup.enter()
let task = session.dataTask(with: request) { (responseData, response, err) in
guard err == nil else {
completion?(.failure(err!))
return
}
guard let jsonData = responseData else {
let err = NSError(domain: "", code: 0, userInfo: [NSLocalizedDescriptionKey : "Data was not retrieved from request"]) as Error
completion?(.failure(err))
return
}
let decoder = JSONDecoder()
do {
let data = try decoder.decode([String: Stock].self, from: jsonData)
let jsonData = data.map{ $0.value }
completion?(.success(jsonData))
dispatchGroup.leave()
} catch {
completion?(.failure(error))
print("Failed to decode using stock URL for \(symbol.symbol ?? ""): \n \(url)")
}
}
task.resume()
}
}
HomeViewController
class HomeViewController: UIViewController, LoginViewControllerDelegate {
override func viewDidLoad() {
super.viewDidLoad()
fetchStocksFromFireStore(completion: { (stocks) -> Void in
APIManager.shareInstance.getStockList(for: self.fetchedStocks) { (result) in
switch result {
case .success(let stocks):
stocks.forEach { (stock) in
print("Company: \(stock.companyName ?? "")")
}
case .failure(let error):
print(error.localizedDescription)
}
}
self.tableView.reloadData()
})
}
}
func fetchStocksFromFireStore(completion: #escaping ([Stock]) -> ()) {
let uid = Auth.auth().currentUser?.uid ?? ""
let db = Firestore.firestore()
db.collection("users").document(uid).collection("stocks").getDocuments { (snapshot, err) in
if let err = err {
print("Error getting stocks snapshot", err.localizedDescription)
return
} else {
snapshot?.documents.forEach({ (snapshot) in
var stock = Stock()
stock.symbol = snapshot.documentID
self.fetchedStocks.append(stock)
})
completion(self.fetchedStocks)
}
}
}
Model
struct Stock {
var symbol: String?
var companyName: String?
var latestPrice: Double?
enum CodingKeys: String, CodingKey {
case symbol
case companyName
case latestPrice
}
enum QuoteKeys: String, CodingKey {
case quote
}
}
extension Stock: Encodable {
func encode(to encoder: Encoder) throws {
var quoteContainer = encoder.container(keyedBy: QuoteKeys.self)
var quoteNestedValues = quoteContainer.nestedContainer(keyedBy: CodingKeys.self, forKey: .quote)
try quoteNestedValues.encode(symbol, forKey: .symbol)
try quoteNestedValues.encode(companyName, forKey: .companyName)
try quoteNestedValues.encode(latestPrice, forKey: .latestPrice)
}
}
extension Stock: Decodable {
init(from decoder: Decoder) throws {
let quoteValues = try decoder.container(keyedBy: QuoteKeys.self)
let quoteNestedValues = try quoteValues.nestedContainer(keyedBy: CodingKeys.self, forKey: .quote)
symbol = try quoteNestedValues.decode(String.self, forKey: .symbol)
companyName = try quoteNestedValues.decode(String.self, forKey: .companyName)
latestPrice = try quoteNestedValues.decode(Double.self, forKey: .latestPrice)
}
}